Seal and method of making same

ABSTRACT

ontact of said two planar surfaces, heating said low melting point metal to its melting point for a sufficient period of time to cause, by capillary force a very thin layer of said metal to appear on and between planar surfaces, such that the tensile strength of said thin layer is greater than its bulk strength, and cooling said metal to solidify same.

United States Patent [1 1 HOCllllli Dec. 4, 1973 SEAL AND METHOD OF MAKING SAME [76] Inventor: Urs E. Hochuli, 7011 Southwork Ter., l-lyattsville, Md.

[22] Filed: Aug. 3, 1970.

[211 App]. N0.: 60,401

[52] US. Cl. 331/945, 29/489 [51] Int. Cl. Hols 3/02 [58] Field of Search 313/220; 220/21 R;

[56] Reierences Cited UNITED STATES PATENTS 2,798,577 7/1957 LaForge, Jr. 174/50.61 3,517,279 6/1970 Ikeda et al. 317/234 3,062,981 1 1/1962 Stoeckert et a1 l74/50.61 3,464,725 9/1969 Bronnes et a1. 174/50.61 2,099,531 11/1937 Passarge 313/331 3,501,013 3/1970 Madsen 29/489 3,029,505 4/1962 Reichenbaum 29/489 3,528,028 I 9/1970 Baird 331/945 3,566,302 2/1971 Rhodes 331/945 Primary Examiner-William L. Sikes AttorneyBeveridge and DeGrandi [57] ABSTRACT Gold films are used as an alloying flux to form 5 micron thick film seals mostly below 300C. Pyrex was sealed to Quartz, ULE*, Cervit*, Germanium, Gallium Arsenide, (GaAs), invar'and copper. The seals can also be used as current feed throughs and as graded seals and are particularly applicable to gas laser devices.

Trademarks 8 Claims, 4 Drawing Figures SEAL AND METHOD OF MAKING SAME BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION The invention disclosed herein is concerned with the sealing and joining of materials with very low thermal expansion coefficients such as Corning Inc., ULE material and Owens-Illinois, Inc. CERVIT material for making ultra stable gas laser devices. However, as will be shown later, it is evident that the sealing method developed covers a far broader range of applications. Such laser structures contain internal mirrors with multilayer dielectric coatings which can be heated to about 400C for periods of about one hour. This represents a thermal upper limit for the sealing temperature and rules out such processes as gold diffusion seals, the AgCL type seal, and probably the well known Mallory process, unless same can be further tailored. (Higher sealing temperatures are of course possible if the user is willing to cool the mirror areas but the introduction of temperature gradients is not very pratical).

The operating temperature of the Helium-Neon laser (referred to hereinafter as the He-Ne laser) is usually below 150C. Reliable seals for periods of time far in excess of hours at the operating temperature of the laser are-necessary. For the outgassing (e.g. vacuum bake-out) purposes it would be convenient to work with seals that are bakable to 300C. The He-Ne laser has mirrors with reflectivities of 99 percent or higher so, for this reason, one cannot tolerate any contamination during the sealing process. He-Ne lasers, using proper cold cathodes, do not need any additional getters. However, the lasers are evacuated to 10 h 10 Torr before filling with the He-Ne gas mixture takes place. It is apparent that seals of the ultra high vacuum type are required. The most severe requirement for frequency stable laser strucures is a constant mirror separation. Since the seals are part of this distance, extremely thin and-stable seals are required.

The present invention while described in terms of its preferred emvironment, namely, the fabrication of ultra stable gas laser structures and devices, is directly broadly to a unique method of making an improved seal or joint, particularly an improved hermetic seal for gas laser devices.

BRIEF DESCRIPTION OF THE DRAWING The above and other features of the invention will become more apparent from the following specification, taken with the accompanying drawing wherein;

FIGS. 1 and 2 disclose a prior art joining sealing technique and FIGS. 3 and 4 illustrate the invention in its preferred form as applied to making ultra-stable laser structures and devices.

BRIEF DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 2, a pair of cup-shaped, quartz, glass or ceramic members 10 and 11 enclosing space 12, are shown .as having prepared mating surfaces or edges 13 and 14, respectively, to which have been applied thin gold films or platings 16 and 17, respectively. While the perimetrical shape may be circular, square or any other shape, for purposes of simplicity, it will be considered as circular. As illustrated, an annular ring on Rib 19 of sealing metal, such as a 0.5mm indium wire metal ring is placed between the gold plated surfaces 16 and 17 and (mild) pressure as indicated by the (arrows) is applied simultaneously with the application of heat to melt the indium rib of ring 19. It will be appreciated that the above procedure may be carried out under a vacuum or in a selected gaseous atmosphere, that the device may be fabricated in air, or a gas filling tubulator, not shown, may be used to vacuum bakeout, the device, and fill same with any desired gas. At any rate the seal of indium metal 198 has a certain finite thickness which can vary with temperature.

In FIG. 3 there is shown the end portions of a laser body member 30, having an electrode 31 in cavity 32 coaxial with discharge bore 33 and connected thereto by a passage (not shown). A plate 35, having reflective coating thereon (not shown) is positioned for sealing to body member 30. The abutting surfaces 38 on the body member and 39 on the plate member are ground smooth as described above and provided with gold plated surfaces 40 and 41, respectively, as also described. (It will be appreciated that such gold coatings are greatly exaggerated in the drawings for purposes of illustration). As shown in FIG. 3, the gold plated ends are butted tightly and a ring of 0.5mm indium wire 45 is placed on the outside. a

The pieces are then heated in a vacuum of 10 Torr or better. (It should be considerably cheaper to use an inert atmosphere for the sealing). It takes about 30 Preferably, mostly, 99.99 percent pure indium is' used. However, the invar to Pyrex seal was made with a 90 percent indium 10 percent silver solder.

SURFACE PREPARATION The surfaces to be joined are polished with a final polish using 5 micron orfiner abrasive compound. Most of the surfaces were finished with Barnsite. The surface flatness is not too critical, a few wavelengths over a distance of 5 cm is sufficient. Both surfaces are then gold coated. The gold coating acts as a wetting agent. For'practical purposes the gold paints that are fired-in yield the most economical coatings. These gold paints do have fluxes added and the exact compositions are usually not released for proprietary reasons. However, a large number of gold paints made by Engelhard Co. have been tested and that companys liquid bright gold No. 7336 has so far'worked satisfactorily. Any

' gold film that is satisfactorily wetted by indium is suitable. Then gold paintsare fired in air to 540C for Pyrex, 750C for ULE, Quartz and CERVIT. Preferably,

- two gold coatings are used, one on each of the abutting SEAL THICKNESS TENSILE STRENGTH Under tension the seals break at about 75 kg for seal areas of 0.54cm This corresponds to 139 kg/cm, which is about 4 times the tensile strength of pure indium.

VAPOR PRESSURE MATERIALS SEALED Endplates of Quartz, ULE and Ge to 30mm Pyrex tubing have been joined. A 5mm thick oxygen free copper plate and a 3mm thick copper plated invar plate was joined to similar Pyrex tubes. In these instances, gold layers were used on the Pyrex only. GaAs CO laser windows were sealed to Pyrex. Many other materials can be sealed by this process.

STABILITY OF THE SEALS Although the thermal expansion coefficient of.indium is unfortunately higher than desired for use in laser devices, this is substantially reduced by the relatively thin seal of the order of 5 microns.

GRADED SEALS The seals described allow one to seal Pyrex tubing directly to Quartz and are therefore useful as graded seals.

CURRENT FEED THROUGH The seals can carry relatively large currents if properly made and can serve as current feed throughs. If this property is not wanted and for eye appeal it is possible to dissolve the gold and indium on the outside in 50 percent HCL and leave just the 5 micron thick film between the joined surfaces.

A NONDESTR UCTIVE SEAL For experimental purposes it is often convenient to disassemble the seals. This is easily done by heating the seals until the indium melts. The seal can then be taken apart,the parts cleaned in aqua regia and reused again.

On materials such as Gallium Arsenide,the gold does alloy and the surface may have to be repolished before it can be reused.

TEMPERATURE CYCLING To get an indication of the seals resistance to thermal stresses, a 2.5mm thick Gallium Arsenide, (GaAs) plate was sealed to the end of a 15mm OD, 13mm ID Pyrex tube. Pyrex was purposedly chosen for a mismatch of the thermal expansion coefficients. (The ductility and low melting point of indium are definitely advantages for joining materials of different thermal expansion coefficients).

The seal was then heated for 3 weeks at C, cooled to room temperature and then dipped into liquid nitrogen. The heating and cooling cycle from 300 to 77K was then repeated three more times. During this complete treatment the seal was connected to a vacuum pump thatenabled us tosee leaks of 10 cm per sec. under STP. No leaks were observed.

HELIUM LEAK TESTS Y To be sure that no very small leaks were overlooked, veeco Ms l2 Helium leak detector was used to test eight different seals. The materials joined included 45mm seals between Corning ULE and 9977W material (two seals), 9mm OD Pyrex capillary to ULE (one seal), 30mm Pyrex tubing to a 5mm thick Cu plate (one seal), 30mm Pyrex tubing to a 3mm thick Quartz plate (one seal), 15mm Pyrex tubing to a 2.5mm thick GaAs plate (the mistreated seal described before), 1 1mm Pyrex tubing to GaAs Brewster angle window (one seal), 30mm Pyrex tubing to a 3mm thick Ge plate. None of the eight seals showed any measurable leaks on the most sensitive scale of about 5.10 cm Helium per sec. under standard temperature and pressure.

What is claimed is:

l. A method ofjoining two body members, comprising forming a planar surface on each of said body members at the place where they are to be joined, each planar surface being constituted by a metal capable of being wetted by a fusible low melting point metal,

placing said surfaces in tight abutting contact with each other, placing a fusible low melting point metal along the line of abutting contact of said two planar surfaces,

heating said low melting point metal to its melting point for a sufficient period of time to cause, by capillary force a very thin layer of said metal to appear on and between planar surfaces, such that the tensile strength of said thin layer is greater than its bulk strength,

and

cooling said metal to solidify same.

2. The method defined in claim 1 wherein said low melting point metal is selected from the group comprising indium, indium-silver; indium-gold alloys.

3. The method defined in claim 1 wherein said low melting point metal is indium and said metal capable of being wetted in gold and each said body member is of another material.

4. The invention defined in claim 3 wherein the thickness of said indium metal between said surfaces is about 5 microns thick.

5. In a gas laser device, a seal made in accordance with the method of claim 4 and wherein the thickness of said indium between said surfaces is about 5 microns thick.

6. A hermetic seal between a pair of body members having a pair of planar, mutually abutting surfaces, each having perimetrical configurations corresponding to that of the seal, comprising i a thin layer of substantially pure indium metal alloyed with metal on at least one of said surfaces and about 5 microns thick such that the tensile 6 members having said abutting surfaces.

8. The invention defined in claim 6 wherein said metal on said at least one surface is gold and the other of said surfaces is a metal selected from the group consisting of gold, copper, silver and platinum.

it i 

2. The method defined in claim 1 wherein said low melting point metal is selected from the group comprising indium, indium-silver; indium-gold alloys.
 3. The method defined in claim 1 wherein said low melting point metal is indium and said metal capable of being wetted in gold and each said body member is of another material.
 4. The invention defined in claim 3 wherein the thickness of said indium metal between said surfaces is about 5 microns thick.
 5. In a gas laser device, a seal made in accordance with the method of claim 4 and wherein the thickness of said indium between said surfaces is about 5 microns thick.
 6. A hermetic seal between a pair of body members having a pair of planar, mutually abutting surfaces, each having perimetrical configurations corresponding to that of the seal, comprising a thin layer of substantially pure indium metal alloyed with metal on at least one of said surfaces and about 5 microns thick such that the tensile strength of said thin layer of indium metal is greater than its bulk strength.
 7. In a gas laser device having body member with a gas chamber formed therein and a reflective member hermetically sealed to one end of said body member, the hermetic seal defined in claim 6 wherein said body member and said reflective member constitute said members having said abutting surfaces.
 8. The invention defined in claim 6 wherein said metal on said at least one surface is gold and the other of said surfaces is a metal selected from the group consisting of gold, copper, silver and platinum. 